Recently, various functions including RFID (Radio Frequency Identification such as wireless identification, NFC (Near Field Communication), a wireless charger, an interactive pen tablet and the like are added to wireless terminal devices including mobile phones and tablet PCs (Personal computers).
NFC means one of electronic tags such as RFID, and refers to a technique for transferring data between terminal devices in close range of 10 cm in a non-contact short-range wireless communication module using a frequency band 13.56 MHz. NFC is extensively used for mobile payment as well as a file transfer method of transferring goods information at a supermarket or general store, or travel information for visitors, traffic information, access control and locking devices, and the like.
In addition, according to the latest Google's announcement, the “Android Beam” equipped with smartphones provides the ability to transfer photos, business cards, files, maps, web sites, etc., as well as mobile payments, as NFC-based local area information transmission and reception functions, from one phone to a different phone.
RFID wireless environments are widely utilized for mobile terminal devices. For example, NFC chips realizing near field communications are mounted in mobile terminal devices, and non-contact smart cards such as USIM (Universal Subscriber Identity Module) cards are also mounted therein. When one of the mobile terminal devices having the NFC chips and the non-contact smart cards is made to access an external RF reader, information of the USIM card in the mobile terminal device is read through near field communications by the RF reader, and necessary information is recorded in the USIM card. By doing so, a built-in function such as an electronic money function is realized.
Information exchanges between the NFC chip and the RF reader are achieved by supplying power for driving the USIM card by an induced electromotive force at 13.56 MHz between a primary coil (or antenna) provided in the RF reader and a secondary coil (or antenna) of the NFC chip mounted in the mobile terminal device.
In general, a non-contact (wireless) charging antenna is mounted in a cover for a battery chamber of a mobile terminal device. Since a late charging circuit connected to an antenna gets small and thus is built in a main body of the mobile terminal device, only antenna parts remain in the battery chamber cover.
The NFC chip mounted in the mobile terminal device may also operate as the external RFID reader, and may be developed to read information recorded in RFID tags and the like. When the NFC chip operates as the RF reader, the antenna (or coil) connected to the NFC chip act as a primary coil to thus transmit power, and an induced electromotive force is generated from the secondary coil (or antenna) mounted in the external RFID tags or the like, to thus realize wireless communications.
That is, a loop antenna of a spiral coil form which can generate an induced electromotive force is needed in order to apply the RFID system or NFC system in the mobile terminal device, the NFC antenna is usually mounted in the battery chamber cover.
In this case, since the induced electromotive force induced in the loop antenna of the spiral coil form is determined by Faraday's law and Lenz's law, it is advantageous that the amount of magnetic flux interlinking with the secondary coil antenna gets large in order to obtain a high voltage signal. The amount of magnetic flux gets large as the amount of a soft magnetic material contained in the secondary coil antenna gets large, and the magnetic permeability of the soft magnetic material becomes high.
In addition, a magnetic field of 100 kHz to several tens of MHz is generated in the antenna coil provided in the mobile terminal device when performing the NFC function with a neighboring mobile terminal device.
Thus, a magnetic shield sheet is essentially used in a mobile terminal device having such an additional function in order to prevent heat generation caused by eddy currents on parts of the mobile terminal device (in particular, a battery) due to the magnetic field, and also to maximize performance of the additional function by focusing the magnetic field.
It is common to use a magnetic material such as an amorphous ribbon, a ferrite, or a polymer sheet containing a magnetic powder as a magnetic field shielding sheet. The magnetic field focusing effect for shielding the magnetic field and improving performance of the additional function may be significant in descending order of the magnetic permeability of an amorphous ribbon, a ferrite, or a polymer sheet containing a magnetic powder.
Korean Patent Registration No. 10-523313 proposed an absorber for a RFID antenna made of a magnetic sheet having a composition selected from the group consisting of Fe—Si—B, Fe—Si—B—Cu—Nb, Fe—Zr—B and Co—Fe—Si—B and including an amorphous alloy, an RFID antenna including the absorber, and an RFID device including the RFID antenna.
The Korean Patent Registration No. 10-523313 discloses a kind of a polymer sheet that is obtained by mixing amorphous alloy powders with a resin to then be made into a sheet form, and has a problem of a low magnetic permeability, that is, the inductance value of the sheet is below 10 μH.
Furthermore, Korean Patent Registration No. 10-623518 disclosed a magnetic sheet for RFID that is configured by laminating a first amorphous alloy ribbon between first and second magnetic sheet layers made of alloy powders including at least one kind of amorphous alloys in order to simplify the manufacturing process of the Korean Patent Registration No. 10-523313 and to increase the magnetic permeability, and compression molding the laminated multi-layer sheet by a rolling or pressing process in order to increase the relative density of the laminated sheet and simultaneously form micro-cracks in the first amorphous alloy ribbon, and an RFID antenna using the magnetic sheet for RFID.
In the Korean Patent Registration No. 10-623518, micro-cracks are formed in the first amorphous alloy ribbon by compression molding the laminated multi-layer sheet, but the micro-cracks have the limitation in lowering the magnetic resistance, to thereby cause a problem that losses due to the eddy current are not greatly reduced.
Meanwhile, a wireless charging antenna is installed in a battery pack of a mobile terminal device, together with an NFC antenna.
In this case, when the transmission rate from the primary side of a wireless charger becomes large, a coupling between adjacent transformers, as well as a defect due to heat generated from peripheral parts, tends to occur. That is, in the case of using a planar coil, the magnetic flux passing through the planar coil is connected to a substrate and the like within the device, and thus the inside of the device generates the heat by eddy currents caused by electromagnetic induction. As a result, large power cannot be transmitted, to thus cause a problem that it takes a long time when charging.
A power reception device of a conventional non-contact charging system is configured to have a magnetic body (or a magnetic sheet) of high magnetic permeability and a large volume arranged on a surface opposite to a primary coil, that is, on the surface of a secondary coil, in order to enhance a coupling for improving the power transmission efficiency, and to improve the shielding performance for heat generation control. According to this arrangement, problems arise in a manner that variation in the inductance of the primary coil is increased, and an operating condition of a resonance circuit is shifted from a resonance condition that can exhibit a sufficient effect according to a relative positional relationship between the magnetic body and the primary coil.
The NFC antenna that currently uses a frequency band of 13.56 MHz is implemented by using a ferrite sheet having a low dependency on frequency.
A ferrite sheet or a polymer sheet containing magnetic powders has rather low magnetic permeability when compared to an amorphous ribbon. In the case of improving the performance of such low magnetic permeability, the ferrite sheet or the polymer sheet becomes thick as compared to the thickness of the amorphous thin ribbon of several tens of μm thick, and thus it is difficult to cope with a trend of a terminal device that is thinned.
In addition, in the case of an amorphous ribbon of high magnetic permeability, the ribbon itself is a metal sheet and thus there is no burden on the thickness. However, problems arise in a manner that an application function is lowered, or a wireless charging efficiency is reduced and heat generation occurs, due to eddy current effects on a ribbon surface when an alternating-current magnetic field on frequency of 100 kHz used for power transmission is applied to the amorphous ribbon.
For the wireless charger, a transmitter for power transmission usually has a structure of employing permanent magnets to help align a receiver, in order to heighten efficiency of the charger at maximum. However, since a thin shielding sheet is magnetized (or saturated) by a direct-current magnetic field of the permanent magnets, performance falls or power transmission efficiency rapidly falls.
Accordingly, in order to exhibit shielding property without being affected by the permanent magnet in the prior art, the thickness of the shielding sheet should be very thick to 0.5 T or thicker. Desired power transmission efficiency can be maintained, only when the thickness of the shielding sheet is very thick to 0.5 T or thicker, to accordingly cause a great obstacle to slimming of a mobile terminal.
Since voltages induced in the NFC antenna and the secondary coil of the wireless charger are determined by Faraday's law and Lenz's law, it is advantageous that the amount of magnetic flux interlinking with the secondary coil gets large in order to obtain a high voltage signal. The amount of magnetic flux gets large as the amount of a soft magnetic material contained in the secondary coil gets large, and the magnetic permeability of the soft magnetic material becomes high. In particular, since NFC and wireless charging are essentially the power transmission by the non-contact, it is required that a magnetic field shielding sheet on which a secondary coil is mounted should be made of a high permeability magnetic material, in order to focus radio frequency electromagnetic waves created in a primary coil of a transmitter on the secondary coil of a receiver.
The conventional magnetic field shielding sheet for NFC and wireless charging does not provide a solution to decrease a heat generation problem due to shielding even with a thin film and to increase wireless charging efficiency. The present inventors have recognized that even if an amorphous ribbon is flake-processed, the inductance (or magnetic permeability) thereof is less reduced and the magnetic resistance thereof is significantly decreased, to accordingly increase a quality factor (Q) of the secondary coil and to reach the present invention.